1. Field of the Invention
The present invention generally relates to a hydraulic power transmission joint to be used for distributing a driving force of a vehicle, and more particularly to a hydraulic power transmission joint which couples two power rotary shafts and transfers a torque corresponding to a difference between the rotational speeds (namely, revolving speeds) thereof.
2. Description of the Related Art
Hitherto, as a conventional hydraulic power transmission joint, there has been known, for example, a joint disclosed in the U.S. Pat. No. 5,103,642. Various torque characteristics such as upped or increased an initial torque, two-stage change of a torque with respect to the difference between the rotational speeds of the power rotary shafts and automatic lock are required for this hydraulic power transmission joint to improve the performance of a vehicle. A valve of FIGS. 1 and 2 for controlling a torque is used to realize such various torque characteristics.
As shown in FIG. 1, a high-pressure chamber 2 communicating with a discharge port (not shown) is formed in a rotary valve 1 of the power transmission joint by plugging an opening thereof with a plug 9. An orifice 10 is made in such a manner as to provide communication from the high-pressure chamber 2 to a low-pressure chamber on the intake-port side (not shown), as illustrated in FIG. 2. A ball valve 3 is enclosed in the high-pressure chamber 2 and is usually pressed by a pin 4 in such a way as to be held in a position as illustrated in these figures. The pin 4 is movably fitted into a passage formed between the high-pressure chamber 2 and an enclosing chamber 7 and is pressed by a spring 6 by way of a ball 5. A stopper pin 8 is enclosed in the spring 6 and is operative to limit the travel of the pin 4 through the ball 5. An oil pressure is low during normal driving, in which the difference between the rotational speeds (or revolving speeds) of the two drive shafts (namely, power drive shafts) is in a range where the torque is equal to or lower than a lock torque Tr corresponding to a small rotational speed difference, the internal oil pressure of the high-pressure chamber 2 is low. Therefore, a pushing force exerted by the spring 6 on the pin 4 through the ball 5, which is designated by an arrow a in FIG. 1, is higher than a pushing force exerted owing to the internal oil pressure of the high-pressure chamber 2 on the pin 4, with the result that the pin 4 holds the ball valve 3 at the position illustrated in these figures. Thus, the orifice 10 opens. Further, oil flows through the orifice 10 as indicated by an arrow b in FIG. 2. In this case, the torque characteristics are represented by a characteristic curve c illustrated in FIG. 5. Namely, in the range d of the rotational-speed difference (or revolving speed difference) in which the torque is equal to or lower than the lock torque Tr, a transfer torque T increases in proportion as the square of a differential revolving speed (namely, a difference between the rotational speeds) .DELTA.N.
In contrast, in the range of the rotational-speed difference (or revolving speed difference) in which the torque is equal to or higher than the lock torque Tr, the pushing force al exerted owing to the internal oil pressure of the high-pressure chamber 2 on the pin 4 becomes higher than the pushing force exerted by the spring 6 on the pin 4. Thus, the pin 4 moves against the force of the spring 6, with the result that the ball valve 3 becomes free and the orifice 10 is closed.
Thereby, a channel or passage from the discharge port to the intake port is closed. As a result, the torque characteristics become those as represented by a graph e of FIG. 6 (namely, an automatic lock torque characteristic), in which the two drive shafts rotate in a body. In the case that the differential revolving speed AN decreases and the torque is lowered to the lock torque Tr and thus the lock is canceled during exhibiting the lock characteristics e, a force becomes needed for pushing the ball valve 3, which has closed the orifice 10, away therefrom. Thust when the oil pressure drops to a value at which a torque Tk equal to or less than the lock torque Tr is transferred, the ball valve 3 opens and the torque having the lock characteristics e is returned to the characteristic region of FIG. 5. At that time, the torque Tk can be arbitrarily set by regulating a position, at which the orifice 10 and the ball valve 3 are in contact with each other, and a position at which the pin 4 and the ball valve 3 are in contact with each other. It is preferable for preventing an occurrence of hunting in the ball valve 3 to cancel the lock when the internal oil pressure of the high-pressure chamber 2 falls to an oil pressure corresponding to the torque Tk which is a little lower than the lock torque Tr.
FIG. 7 illustrates another conventional hydraulic power transmission joint, the rotary valve 11 of which is a valve mechanism for controlling the torque characteristics. A high-pressure chamber 12 communicating with a discharge port of a plunger piston is formed in a surface portion of the rotary valve 11. Further, a communicating groove 13 for providing communication between or among a plurality of high-pressure chambers is formed in the other surface portion of the rotary valve 11. Further, an enclosing chamber 14, whose opening is closed by a plug 18, is formed in the rotary valve 11. Moreover, a ball valve 15 is enclosed in the enclosing chamber 14. The enclosing chamber 14 communicates with the high-pressure chamber 12 through an orifice 16. The ball valve 15 is pressed by a spring 17 against an aperture thereof communicating with the orifice 16. Further, a discharge port 19 is drilled through the rotary valve 11 in such a way as to communicate with the enclosing chamber 14, for the purpose of relieving the internal oil pressure of the enclosing chamber 14. When there is substantially no rotational-speed difference between the two drive shafts, the spring 17 presses the ball valve 15 against the aperture of the orifice 16, so that the orifice 16 is closed. When the rotational-speed difference therebetween increases and thus the internal oil pressure of the high-pressure chamber 12 rises to a value equal to or higher than the predetermined value, the ball valve 15 moves against a force exerted by the spring 17 and opens the orifice 16. At that time, the torque characteristics become the characteristic as illustrated in FIG. 8, so that an initial torque can be increased by a torque g.
Further, various torque characteristics are required for this hydraulic power transmission joint to improve the performance of a vehicle. The combination of an increased-initial-torque characteristic as illustrated in FIG. 8 and a two-stage torque characteristic as illustrated in FIG. 9 is required for improving, for example, the stability or road-holding of a vehicle running on what is called a low-.mu. road. surface, the coefficient of friction of which is low. Further, an automatic lock torque characteristic as illustrated in FIG. 6 is required for enhancing the road ability of the vehicle. Moreover, the torque limiter characteristic (namely, the torque characteristics of a torque limiter) as illustrated in FIG. 10 is required for reducing the size and weight of a power train system.
However, although the conventional valve mechanism for controlling the torque characteristics can obtain the automatic lock characteristic of FIG. 6 and the increased-initial-torque characteristic of FIG. 8, each of such characteristics can be obtained only as a separate characteristic. Further, the conventional valve mechanism can obtain neither the second-stage torque characteristic of FIG. 9 nor the torque limiter characteristic of FIG. 10. Moreover, the torque characteristic of FIG. 11 acting as the combination of the automatic lock torque characteristic and the increased-initial-torque characteristic cannot be obtained. Furthermore, a valve structure for obtaining the automatic lock torque characteristic of FIG. 6 becomes complex as illustrated in FIG. 2. Thus, there has been a problem that the manufacturing cost is increased.
Further, in the case of the valve structure of FIGS. 1 and 2 for realizing the automatic lock characteristic or performance, the gap between the pin 4 and the hole, into which the pin 4 is inserted, is set to be small in order to maintain the internal oil pressure of the high-pressure chamber 2. Thus, there has been a problem that the gap formed therebetween is clogged with foreign particles-contained in oil and as a result, the pin 4 comes to be unable to move. Moreover, it is difficult owing to the small diameter of this hole to secure the accuracy. Furthermore, the ball valve 3 blocks the orifice 10 to thereby lock the rotary valve. At that time, the ball valve 3 strikes hard against an aperture portion of the orifice 10. Thus, there has been a problem that in such a case, the aperture portion of the orifice 10 often brakes and consequently, the orifice cannot be closed by the ball valve 3. Especially, the rotary valve 1, in which the orifice 10 is formed, is made of a sintered material. Therefore, the hardening of the rotary valve to be performed by utilizing a heat treatment or the like for facilitating the working thereof cannot be implemented. Thus the orifice 10 is liable to be broken by a collision thereof with the ball valve 3. To solve this problem, a hard material may be used as the material of the rotary valve 1. However, in this case, the working of such a rotary valve is difficult. Consequently, the manufacturing cost thereof is increased.